add DO persister tests, fix prefix parsing, fix empty storage handling

[bitmage]

Summary

Fixes two bugs in the Durable Object Storage persister when using a non-empty storagePrefix:

1. Prefix parsing bug (line 46): slice(key, storagePrefix.length, 1) returns empty string when prefix length > 1. Fixed to slice(key, storagePrefix.length, storagePrefix.length + 1).
2. Empty storage handling (line 103-105): getPersisted() returned valid-but-empty content [[{}, '', 0], [{}, '', 0]] when storage was empty, causing load() to wipe existing store data. Now returns undefined when no data exists.

How did you test this change?

There were formerly no tests (!) for this persistence provider. I added a mock structure which allows it to be tested in a way that I believe is idiomatic with the other providers. Initially the first bug was my target, but I discovered the second when I got the test suite to run.

[jamesgpearce]

👀 wow, thanks! I'll get these merged in this week.